1. Field of the Invention
The present invention relates generally to the manufacture of optical waveguide fibers, and in particular to manufacturing a fluorine doped preform from which an optical fiber may be drawn.
2. Technical Background
Optical fibers having a fluorine doped region have unique attributes for long haul optical fibers, dispersion compensating optical fibers, dispersion slope compensating optical fibers, and high data rate optical fibers. The ability to include fluorine in an optical waveguide preform is an important aspect of producing an optical fiber with a fluorine doped region.
Prior attempts to incorporate fluorine into a preform include depositing fluorine doped soot on a starting member or bait rod. Typically, the starting member is a sintered core cane. One of the drawbacks of this approach is that the deposited fluorine exhibits significant migration from the region or regions of interest and migration into areas not intended to include the dopant. Preforms fluorinated during deposition have also exhibited a fluorine loss of between forty percent (40%) to fifty percent (50%) during consolidation. One reason for the low retention rate of fluorine is the production of the compound SiF4 during deposition. Typically, SiF4 generated during deposition will volatilize from the preform during consolidation. As a result, the time of consolidation must be extended in an effort to redope the preform with SiF4.
The relatively long times at the relatively low temperatures of the slow ramp consolidation impact fluorine retention in at least two ways: (1) the fluorine containing vapor (mainly SiF4) evolving from the soot has sufficient time to diffuse out of the preform; and (2) the equilibrium of redoing the preform with SiF4 vapor is not a favored reaction at the lower temperatures and low ramp rates. Thus, deposition of fluorinated soot with a redoing step has not proven to be effective.
Fluorine may also be added to a soot preform during a consolidation doping step as taught in Berkeley U.S. Pat. No. 4,629,485. In one such consolidation doping process, soot is deposited on a core cane forming a physical interface between a central core region of the optical fiber and the soot region. The soot coated, core cane is dried in a 2% chlorine containing atmosphere for approximately 2 hours at 1000° C. The dried preform is then exposed to a fluorine containing atmosphere for 1-4 hours at a temperature of between 1100° C. and 1400° C. The fluorine doped preform is then fully sintered. Subsequently, the preform is drawn into an optical fiber. This method adds unnecessary time and steps to the manufacturing of the preform, as well as additional costs thereto.
While doping has been demonstrated during the consolidation process, fluorine doping during lay down is preferred as it is possible to make more complex profiles in a reduced number of steps. As noted above, fluorine doping during laydown is plagued with the problem of fluorine migration into areas where fluorine doping is not intended. The use of glass barrier layers has been used to prevent the migration of fluorine. However, the use of glass barrier layers on both sides of a fluorine doped region may cause water to be trapped within the contained layer. Any water within the contained layer that cannot be removed using conventional drying procedures, i.e., since it is trapped between the glass barrier layers, may lead to an unacceptably high attenuation within the resultant optical fibers.
A need exists for alternative methods to produce preforms having at least one fluorine doped region which does not exhibit significant migration of fluorine in the preform or high attenuation in the fiber.